Prioritizing binary mixtures of metals with organic micropollutants in European freshwater: who drives the risk?

The toxicity of metal-organic mixtures and their environmental occurrence is currently poorly understood. This study aimed to identify and prioritize organic substances that potentially pose an aquatic mixture risk with metals. To guide and facilitate future experimental research, the focus was on binary mixture combinations alone. Monitoring data of European freshwaters was used to assess 12,180 possible binary mixture combinations of 609 unique organic substances and 20 metals. To evaluate the risk of metal-organic mixtures on an ecosystem level, a generic risk quotient approach using predicted no-effect concentrations or equivalent values was performed. The approach identified 105 binary priority combinations with 55 priority organic substances and 12 priority metals. For all priority combinations, more than 10% of measured samples exceeded the defined risk threshold of 1. To understand the risk of the 55 priority organic substances to specific target species, a toxic unit approach was performed using chronic toxicity data for Daphnia magna, Raphidocellis subcapitata, and fish species. To D. magna and fish, 14 out of the 55 priority substances posed a risk, all substances with various modes of action (MoA). To D. magna, the insecticide methomyl, an acetylcholine esterase inhibitor, was of highest priority. For fish, the fatty acid inhibitor triclosan was of highest priority. To R. subcapitata, 13 out of the 55 priority substances posed a risk in binary pairs with metals. These 13 substances were mainly herbicides, dominated by photosynthesis-inhibiting and fatty acid synthesis-inhibiting MoA. The prioritized binary mixtures are intended to assist the design of future laboratory studies of metal organic micropollutant mixtures across a range of MoA.